Carbon nanosphere with at least one opening, method for preparing the same, carbon nanosphere-impregnated catalyst using the carbon nanosphere, and fuel cell using the catalyst

ABSTRACT

A carbon nanosphere has at least one opening. The carbon nanosphere is obtained by preparing a carbon nanosphere and treating it with an acid to form the opening. The carbon nanosphere with at least one opening has higher utilization of a surface area and electrical conductivity and lower mass transfer resistance than a conventional carbon nanotube, thus allowing for higher current density and cell voltage with a smaller amount of metal catalyst per unit area of a fuel cell electrode.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority to and the benefit of Korean PatentApplication No. 10-2004-0088218, filed on Nov. 2, 2004 in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a carbon nanosphere with at least oneopening, a method of preparing the same, a supported catalyst, and afuel cell, comprising the same. In particular, the invention relates toa carbon nanosphere with at least one opening, a method of preparing thesame, a supported catalyst in which metal catalyst particles aresupported on the carbon nanosphere with at least one opening, and a fuelcell that uses the supported catalyst.

2. Description of the Related Art

Fuel cells are clean energy sources that have received considerableinterest as fossil fuels alternatives. A fuel cell is a power generatingsystem that produces direct current electricity through anelectrochemical reaction between fuel such as hydrogen, natural gas, ormethanol and an oxidizing agent.

In general, a fuel cell includes an anode (fuel electrode) where asupplied fuel is oxidized, a cathode (air electrode) where the oxidizingagent is reduced, and an electrolyte membrane that is interposed betweenthe anode and the cathode and provides a path for transporting ions thatare produced at the anode to the cathode. Electrons are generatedthrough the oxidation of the fuel at the anode, which then flow via anexternal circuit and are returned to the cathode to reduce the oxidizingagent.

Among the most important features of the fuel cell are catalysts thatare present at the anode and the cathode to catalyze the reactions thatoccur at the electrodes. Thus, many experiments have been conducted toincrease the activity of the catalysts used in the electrodes. Thecatalytic activity increases as the reaction surface area of thecatalyst increases, and thus, the reaction surface area may be increasedby decreasing the particle diameter of the catalyst to uniformlydistribute the catalyst on the electrodes.

Conventionally, a platinum (Pt) catalyst, for example, is applied to acarbon cloth, etc. However, the catalyst cannot be uniformly dispersedon the carbon and the surface area and electrical conductivity of thecarbon support, etc., are not sufficiently high.

A metal catalyst that is supported on porous carbon powders has beensuggested. The specific surface area of the porous carbon powders can becontrolled such that the ability to impregnate a catalyst is high.However, when the carbon powders are graphited or crystallized toincrease the electrical conductivity, the structures of the carbonpowders are altered. In addition, the electrical conductivity cannot beincreased sufficiently. Thus, the surface properties of the carbonpowders must be improved.

In order to overcome these problems, the use of carbon nanotubes ornanohorns as supports has been suggested and a significant amount ofresearch has been done in this area.

Carbon nanotubes are very fine cylindrical materials that have adiameter of about several nm to about several tens of nm, a length ofabout several μm to about several hundreds of μm, high anisotropy, andcome in various structures and shapes such as single-walled,multi-walled, or rope shapes. In carbon nanotubes, one carbon atom bondsto three other carbon atoms to form a hexagonal honeycomb (a pentagonalor heptagonal honeycomb may be formed depending on the curvature radiusat the bounding position of the carbon atom). Carbon nanotubes may havemetallic or semiconductor properties depending on their structures, aremechanically strong (about 100 times stronger than steel), have chemicalstability, high thermal conductivity, and a hollow structure. Thus,carbon nanotubes may be used in various microscopic and macroscopicapplications, such as catalyst carriers.

Carbon nanotubes have high electrical conductivity and thus, canincrease the utilization of the electrical energy that is generatedduring an electrochemical reaction. However, a catalyst can be supportedonly on the outer walls of carbon nanotubes and the surface area onwhich the catalyst can be substantially supported is small, relative tothe total surface area of the carbon nanotubes. That is, the capabilityto impregnate a catalyst is low. Further, when the carbon nanotubes havea high aspect ratio, they cannot be uniformly dispersed easily on thesurface of an electrode during formation of the electrode. Inparticular, the diffusion resistance of the material is high due to theclosed structures of their ends, which is one of the largest obstaclesto their use as a catalyst carrier.

In order to overcome the problems, the use of carbon nanohorns has beensuggested. Carbon nanohorns have a conical structure, similar to ends ofnanotubes that are cut off from the nanotubes. The carbon nanohorns arevery short and a catalyst can be impregnated even in their innermostregions. However, carbon nanohorns have an internal diameter of about 1nm and the optimal particle size of the catalyst is about 2-3 nm. Thus,the catalyst cannot be sufficiently supported on the carbon nanohorns.In this case, the catalyst is supported only on the outer walls of thenanohorns and the advantage of the high surface area of the nanohorns islost. Further, a nanohorn has one closed end, and thus, when nanohornsare used as catalyst supports, a fuel cannot flow smoothly, resulting ina low catalytic efficiency.

In order to overcome these problems, the use of short carbon nanotubesthat have open ends has been suggested. However, since the carbonnanotubes are flexible and resistant to an applied stress, short carbonnanotubes that have open ends cannot be prepared easily.

Methods for cutting a carbon nanotube in order to prepare a short carbonnanotube that has open ends have been suggested. One method includesusing ultrasonic waves (K. L. Lu et al., Carbon 34, 814-816 (1996); K.B. Shelimov et al., Chem. Phys. Lett. 282, 429-434 (1998); J. Liu etal., Science 280, 1253-1256 (1998)). However, the resulting short carbonnanotubes have a low yield and an inconsistent relative length. Anothermethod includes using an STM voltage (L. C. Venema et al., Appl. Phys.Lett. 71, 2629-2631 (1999)). In this method, the resulting short carbonnanotubes do not have open ends. An additional method includes usingball milling, but short carbon nanotubes that have both ends open cannotbe produced.

A conventional carbon nanotube can be processed into a short carbonnanotube with both ends open by a mechanical or chemical treatment, butthe processing cannot be performed easily due to a strong binding forcebetween crystalline carbons.

SUMMARY OF THE INVENTION

The present invention provides a carbon nanosphere that has at least oneopening that has a higher surface area utilization, higher electricalconductivity, and lower mass transfer resistance than a conventionalcarbon nanosphere or carbon nanotube. This allows for higher currentdensity and cell voltage with a smaller amount of metal catalyst perunit area of a fuel cell electrode.

The present invention also provides a method for preparing the carbonnanosphere with at least one opening in a simple and efficient manner.

Additional features of the invention will be set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention.

The present invention discloses a carbon nanosphere with at least oneopening.

The present invention also discloses a method for preparing a carbonnanosphere with at least one opening comprising heating an organic metalcompound in a preheating region, heating the resulting organic metalcompound, a dilution gas, and a hydrocarbon gas in a reaction furnace togrow a carbon nanosphere, and treating the resulting carbon nanospherewith an acid.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a flow chart for a method of preparing a carbon nanospherewith at least one opening according to an exemplary embodiment of thepresent invention.

FIG. 2A, FIG. 2B, FIG. 3A and FIG. 3B are transmission electronmicroscopic (TEM) photos of carbon nanospheres that have at least oneopening according to an embodiment of the present invention.

FIG. 4 is an X-ray diffraction (XRD) graph of a metal catalyst supportedon a carbon nanosphere that has at least one opening according to anexemplary embodiment of the present invention.

FIG. 5 is a graph of current density vs. cell potential for a fuel cellaccording to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The invention is described more fully hereinafter with reference to theaccompanying drawings, in which embodiments of the invention are shown.This invention may, however, be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure isthorough, and will fully convey the scope of the invention to thoseskilled in the art. In the drawings, the size and relative sizes oflayers and regions may be exaggerated for clarity.

It will be understood that when an element such as a layer, film, regionor substrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” another element, there are no intervening elements present.

According to an exemplary embodiment of the present invention, a methodfor preparing a carbon nanosphere that has at least one opening isprovided. In this method, a carbon nanosphere is prepared using anyworkable method, and then the obtained carbon nanosphere is treated withan acid to form the opening. In order to obtain the carbon nanospherewith a desired diameter and wall thickness, ratios of flow rates ofsupplied reactants may be controlled.

Methods for preparing a carbon nanosphere may include, but are notlimited to arc discharge, laser vaporization, thermochemical vapordeposition, and plasma enhanced chemical vapor deposition, etc.

When using the arc discharge method and the laser vaporization method,the yield of carbon nanotubes is relatively low. In addition, a largenumber of amorphous phase carbon clusters are produced along with thecarbon nanotube, which requires a complicated purifying process. It isalso difficult to grow a large amount of carbon nanotubes on a substratewith a large area.

Chemical vapor deposition produces vertically arranged carbon nanotubesthat can be synthesized at a high purity and a high yield. However, itis very difficult to control the diameter and length of a carbonnanotube. The length of the carbon nanotube is about several μm toseveral tens of μm, which is not suitable for a catalyst carrier.

Accordingly, in order to prepare a carbon nanosphere using theconventional chemical vapor deposition method, operation parameters mustbe altered. For example, the growth temperature is lowered and theresidence time is reduced.

When the growth temperature of the carbon nanosphere is low, an organicmetal compound used as a reactant in preparing the carbon nanosphere maydecompose at a relatively low temperature. For example, the organicmetal compound may be copper tartrate. The copper tartrate includes amonovalent copper compound (Cu₂C₄H₄O₆) and a divalent copper compound(CuC₄H₄O₆), i.e., copper (II) tartrate. In many cases, an organic metalcompound such as copper (II) tartrate is generally present in the formof a hydrated salt. Copper (II) tartrate may include DL-tartrate andL-tartrate, which are dihydrated salts, and mesotartrate, which is atrihydrated salt.

Referring to FIG. 1, an organic metal compound 10 is preheated in apreheating region 20 to evaporate hydrated water molecules. Anotherpurpose of the preheating is to control the size of the coppernanoparticles or copper nanospheres by adjusting the growth temperatureand the residence time. The temperature of the preheating region 20molecules may be about 100° C. to about 200° C. If the temperature inthe preheating region 20 is less than 100° C., the crystalline watermolecules, i.e., the hydrated water molecules cannot evaporatesufficiently. If the temperature in the preheating region 20 is greaterthan about 200° C., the organic metal compound 10 may decomposeexcessively, thereby forming larger metal particles.

The residence time of the organic metal compound 10 in the preheatingregion 20 may be about 20 minutes to about 40 minutes. If the residencetime is less than about 20 minutes, the crystalline water molecules,i.e., the hydrated water molecules, cannot evaporate sufficiently. Ifthe residence time is greater than about 40 minutes, the organic metalcompound 10 may decompose completely, and thus the metal in the organicmetal compound 10 may form into particles.

Water molecules thus evaporated in the preheating region 20 may beremoved using a water removal unit such as a cooling condenser.

The organic metal compound 10 from which water was removed in thepreheating region 20, a dilution gas 12, and a hydrocarbon gas 14 aretransferred to a reaction furnace 22, in which a reaction for generatinga carbon nanosphere 16 is performed. The hydrocarbon gas 14 may be anyhydrocarbon gas that is used in preparing carbon nanotubes such asmethane, ethylene, or acetylene (C₂H₂). The dilution gas 12 may benitrogen. In the reaction furnace 22, the hydrocarbon gas 14 isthermally decomposed by the copper nanoparticle to form the carbonnanosphere 16.

The temperature in the reaction furnace 22 may be about 450° C. to about600° C. If the temperature in the reaction furnace 22 is less than about450° C., the metal cannot be completely decomposed from the organicmetal compound 10, thus preventing catalytic action. Further, thedecomposition of carbon is too slow, and so the structure of the carbonnanosphere 16 cannot be formed completely. If the temperature in thereaction furnace 22 is greater than about 600° C., the decomposition ofcarbon occurs too quickly and the number of walls of the carbonnanosphere 16 increases, resulting in an excessively thick carbonnanosphere, although the crystallinity increases.

The mean residence time of the above reactants in the reaction furnace22 may be about 20 to about 40 minutes. If the mean residence time inthe reaction furnace 22 is less than about 20 minutes, the structure ofthe carbon nanosphere 16 cannot be formed completely. If the meanresidence time in the reaction furnace 22 is greater than about 40minutes, the number of walls of the carbon nanosphere 16 increases,resulting in an excessively thick carbon nanosphere.

Since the residence time of the reactants in the reaction furnace 22 isshorter than the residence time in the conventional method as describedabove, defects are generated in C—C bonds in the carbon nanosphere 16.Defects are generated because the environments in which the carbonnanosphere 16 is grown change rapidly as the carbon nanosphere 16 passesthrough the preheating region 20 and the heating furnace 22. Thus,crystals with a constant size cannot be formed.

Although some of the defects may be opened during the growth of thecarbon nanosphere 16, most of the defects are generally opened bytreating the carbon nanosphere 16 with a strong acid to oxidize thedefective portions. There may be about one, two, three or more defects.When one defect is present in the carbon nanosphere 16, after the defectis oxidized, the carbon nanosphere 16 is in the shape of a pot with anopen hole. When two defects are present in the carbon nanosphere 16,after the defects are oxidized, the carbon nanosphere 16 is in the shapeof a short tube with an expanded middle portion.

When one opening is present in the carbon nanosphere 16, a flow enteringthe carbon nanosphere 16 and a flow exiting the carbon nanosphere 16share the same space, thus, mass transfer may be less than when at leasttwo openings are present. However, since only a small portion of thetotal carbon nanospheres 16 have only one opening, the presence ofcarbon nanospheres with only one opening does not have a decisive effecton the overall physical properties of the carbon nanospheres 16.

As described above, in order to obtain a carbon nanosphere 16 that has adesired diameter and wall thickness, the ratios of the flow rates ofreactants must be suitably controlled.

The molar flow rate of the hydrocarbon gas 14 may be about 0.0006 toabout 0.0025 times the molar flow rate of the dilution gas 12. If themolar flow rate of the hydrocarbon gas 14 is less than about 0.0006times the molar flow rate of the dilution gas 12, the amount of thecarbon required to form the structure of the carbon nanosphere 16 isinsufficient, and thus the desired carbon nanosphere 16 cannot be formedcompletely. If the molar flow rate of the hydrocarbon gas 14 is greaterthan about 0.0025 times the molar flow rate of the dilution gas 12, toomuch carbon is decomposed, and the wall thickness of the carbonnanosphere 16 may be too thick.

The amount and the structure of the carbon nanosphere 16 that is formeddepend on the flow rates of the dilution gas 12 and the hydrocarbon gas14 and the residence time in the reaction furnace 22, regardless of theconcentration of the organic metal compound that is 10 used, as long asthere is a specific minimum amount of the organic metal compound 10.That is, when a sufficient amount of the organic metal compound 10 ispresent in the reaction furnace 22, the amount and the structure of thecarbon nanosphere 16 that is formed depends mainly on the flow rates ofthe dilution gas 12 and the hydrocarbon gas 14 and the residence time inthe reaction furnace 22. Thus, a sufficient amount of the organic metalcompound 10 may be introduced into the reaction furnace 22 beforestarting the reaction. The reaction may end when a residual amount ofthe organic metal compound 10 falls to less than the required minimumamount due to consumption during the reaction. Alternatively, an excessamount of the organic metal compound 10 may be introduced into thereaction furnace 22 together with the other reactants.

When the organic metal compound 10 is copper tartrate, the requiredminimum concentration is 12.5 g of copper tartrate per gram of carbonnanosphere 16 formed. Thus, when the copper tartrate is to be introducedinto the reaction furnace 22 before starting the reaction, the weight ofthe copper tartrate must be at least 12.5 times the weight of the carbonnanosphere 16 to be formed. When an excess of the copper tartrate isintroduced into the reaction furnace 22 with the other reactants, theflow rate of the copper tartrate must be at least 12.5 times theformation rate of the carbon nanosphere 16.

The concentration of the copper tartrate supplied may be about 12.5 g toabout 100 g per gram of the carbon nanosphere 16 formed. If theconcentration of the copper tartrate supplied is less than about 12.5 gper gram of the carbon nanosphere 16 formed, a carbon nanosphere withthe desired structure cannot be obtained. If the concentration of thecopper tartrate supplied is greater than about 100 g per gram of thecarbon nanosphere 16 formed, the size of the reaction furnace 22 shouldbe increased and production costs may increase.

Since the carbon nanosphere 16 obtained in the reaction furnace 22 doesnot have an opening, it has a small surface area, a high mass transferresistance, and cannot impregnate many metal catalyst particles. Inorder to form opening son a surface of the carbon nanosphere 16, thecarbon nanosphere 16 must be treated with a strong acid in the acidtreatment region 24. When the carbon nanosphere 16 is treated with theacid, defects are first oxidized, thereby forming openings on thesurface of the carbon nanosphere 16. As a result, a carbon nanosphere 18with at least one opening is obtained. The acid may include, but is notlimited to nitric acid.

A carbon nanosphere with at least one opening may be prepared using themethod as described in the previous embodiment. The carbon nanospheremay have openings all over its surface and a diameter of about 100 nm toabout 350 nm and a wall thickness of about 5 nm to about 30 nm. Each ofthe openings may have a diameter of about 5 nm or greater, particularlyabout 5 to about 150 nm.

If the diameter of the carbon nanosphere with at least one opening isless than about 100 nm, a space into which the catalyst may penetrate istoo small, thereby decreasing the utilization of the catalyst. If thediameter of the carbon nanosphere that has at least one opening isgreater than about 350 nm, the specific surface area is decreased andthe carbon nanosphere may not effectively function as a catalystcarrier.

If the wall of the carbon nanosphere with at least one opening is lessthan about 5 nm thick, the mechanical strength is not sufficient. If thewall of the carbon nanosphere with at least one opening is greater thanabout 30 nm thick, the openings cannot be formed easily.

If the diameter of each of the openings is less than about 5 nm, thecatalyst particles may not easily enter the openings of the carbonnanosphere and may not be uniformly distributed in the inside andoutside of the carbon nanosphere. Further, good mass transfer is notattained, thereby decreasing the catalytic efficiency. If the diameterof each of the openings is greater than about 150 nm, the specificsurface area decreases and the carbon nanosphere may not effectivelyfunction as a catalyst carrier.

The metal catalyst may be supported on the carbon nanosphere that has atleast one opening using any workable method. The metal catalystparticles to be supported may have an average particle size of about 2nm to about 5 nm. If the average particle size of the metal catalystparticles is less than about 2 nm, the metal catalyst particles cannotsufficiently provide an active site for the catalytic reaction. If theaverage particle size of the metal catalyst particles is greater thanabout 5 nm, the specific surface area decreases, thereby reducing thecatalytic efficiency.

The metal catalyst particles used in the carbon nanosphere-supportedcatalyst are not specifically limited, but may include platinum (Pt) ora Pt alloy when the supported catalyst is used in a proton exchangemembrane fuel cell (PEMFC) or a direct methanol fuel cell (DMFC). The Ptalloy may include Ti, Cr, Mn, Fe, Co, Ni, Cu, Ga, Zr, Hf, Ru, Ir, Rh,Pd, Os, or a mixture thereof. In the DMFC, methanol is oxidized togenerate carbon monoxide, which causes poisoning of a Pt catalyst. Toprevent this poisoning, the Pt alloy catalyst may be used.

The carbon nanosphere-supported catalyst may be used as an activecomponent in a fuel cell electrode. The electrode for the fuel cell maybe prepared using any workable method. For example, the carbonnanosphere-impregnated catalyst may be dispersed in a solution of anionomer such as Nafion® and then combined with isopropyl alcohol toprepare a slurry. The slurry is coated on a waterproof carbon paperusing spray coating and then dried to obtain the electrode.

A fuel cell according to an exemplary embodiment of the presentinvention may be a PEMFC, a DMFC, etc. Fuel cells may be classified intoalkaline, phosphoric acid, molten carbonate, solid oxide, and solidpolymer electrolyte fuel cells depending on the type of electrolyteused. In particular, when a Pt catalyst is used, the carbonnanosphere-supported catalyst is suitable for alkaline, phosphoric acid,and solid polymer electrolyte fuel cells. Since the DMFC has the samestructure as the solid polymer electrolyte fuel cell, the carbonnanosphere-supported catalyst may also be used in the DMFC. Since aliquid fuel may diffuse efficiently through the inside of the carbonnanosphere that has at least one opening, the carbonnanosphere-supported catalyst is particularly suitable for the DMFC.

Hereinafter, the present invention will be described in more detail withreference to the following examples. However, these examples are givenfor the purpose of illustration and are not intended to limit the scopeof the invention.

EXAMPLES 1-3

A reaction furnace was purged with a dilution gas at a volumetric flowrate of 500 standard cubic centimeters per minute (sccm) underatmospheric pressure while the temperature of the reaction furnace waselevated to the temperatures listed in Table 1. Copper tartrate(CuC₄H₄O₆.H₂O) was used as an organic metal compound and acetylene wasused as a hydrocarbon gas to form carbon nanospheres. The flow rate ofthe dilution gas was 16 L/min and the resulting carbon nanospheres weretreated with nitric acid to obtain carbon nanospheres that have at leastone opening. The reaction conditions of Examples 1-3 are shown in Table1.

TABLE 1 Temperature (° C.) Residence Time (min) Ramp Flow rate ofPreheating Reaction Preheating Reaction rate hydrocarbon Dilution regionfurnace region furnace (° C./min) gas (ml/min) gas Example 1 200 450 2030 12.5 10 Argon Example 2 150 500 30 20 17.5 30 Nitrogen Example 3 100600 40 30 20 20 Nitrogen

The carbon nanospheres that have at least one opening that were preparedunder the conditions described in Table 1 exhibited the physicalproperties listed in Table 2.

TABLE 2 Diameter of carbon Wall thickness nanosphere (nm) (nm) Example 1200 10 Example 2 250 25 Example 3 300 20

FIG. 2A and FIG. 2B are transmission electron microscopic (TEM) photosof the carbon nanospheres obtained in Examples 1-3. It was confirmedfrom FIG. 2A and FIG. 2B that each of the obtained carbon nanosphereshad openings.

EXAMPLES 4 and 5

In order to confirm the effects of the reaction conditions on the shapeof carbon nanosphere, experiments were preformed by changing thereaction conditions. Copper tartrate (CuC₄H₄O₆.H₂O) was used as anorganic metal compound and acetylene was used as a hydrocarbon gas. Theflow rate of the dilution gas was 16 L/min and the resulting carbonnanospheres were treated with nitric acid. The reaction conditions ofExamples 4 and 5 are shown in Table 3.

TABLE 3 Temperature (° C.) Residence Time (min) Ramp Flow rate ofPreheating Reaction Preheating Reaction rate hydrocarbon Dilution regionfurnace region furnace (° C./min) gas (ml/min) gas Example 4 150 700 3030 15.7 40 Argon Example 5 100 750 40 30 16.3 25 Nitrogen

The carbon nanospheres with at least one opening that were preparedunder the conditions described in Table 3 exhibited the physicalproperties listed in Table 4.

TABLE 4 Diameter of carbon Wall thickness nanosphere (nm) (nm) Example 4500 35 Example 5 600 30

FIG. 3A and FIG. 3B are TEM photos of the carbon nanospheres of Example4 and Example 5, respectively. The resulting carbon nanospheres hadopenings, but the openings were insufficiently formed relative to theopenings in the carbon nanospheres obtained in Examples 1-3.

Specifically, it was confirmed from the results in Table 2 and Table 4that the carbon nanospheres obtained in Examples 1-3 had smaller wallthicknesses than the carbon nanospheres obtained in Examples 4 and 5.When the wall thickness is small, the openings can be well-formed duringthe acid treatment. Since the carbon nanospheres obtained in Examples1-3 had thinner walls than the carbon nanospheres obtained in Examples 4and 5, it was expected that the carbon nanospheres obtained in Examples1-3 had more openings than the carbon nanospheres obtained in Examples 4and 5.

It was confirmed from the TEM photos in FIG. 2A, FIG. 2B, FIG. 3A andFIG. 3B that the openings were sufficiently formed in the carbonnanospheres obtained in Examples 1-3, while the openings were lessformed in the carbon nanospheres obtained in Examples 4 and 5. Thus, itis believed that whether the openings are well formed or not depends onthe temperature in the reaction furnace, the residence time, and/or theratios of flow rates of the reactants.

EXAMPLE 6

0.5 g of the carbon nanosphere with at least one opening obtained inExample 1 as a carbon support was placed in a plastic bag, and then1.9236 g of H₂PtCl₆ was dissolved in 1.5 mL of acetone in a beaker. Theacetone solution was mixed with the carbon carrier in the plastic bag.The mixture was dried in air for 4 hours, and was then transferred to acrucible and dried in a drier at 60° C. overnight. Then, the cruciblewas placed in an electric furnace under nitrogen flow for 10 minutes.Next, nitrogen gas was replaced with hydrogen gas and the temperature inthe electric furnace was raised from room temperature to 200° C. andmaintained for 2 hours to reduce a Pt salt that was impregnated on thecarbon support. The hydrogen gas was replaced with nitrogen gas and thetemperature in the electric furnace was raised to 250° C. at a rate of5° C./min and maintained at 250° C. for 5 hours, and then cooled to roomtemperature. Thus, a carbon nanosphere-supported catalyst with a loadingof 60 wt % of Pt was obtained.

An X-ray diffraction (XRD) graph for the obtained carbonnanosphere-supported catalyst is shown in FIG. 4. An average particlesize of the metal particles impregnated on the support was 3.2 nm.

Preparation of a Fuel Cell

The carbon nanosphere-supported catalyst prepared in Example 6 wasdispersed in a dispersion of Nafion 115® (DuPont) in isopropyl alcoholto prepare a slurry. The slurry was coated on a carbon electrode using aspray process to obtain 1 mg/cm² of the coated catalyst based on a Ptloading. Then, the electrode was passed through a rolling machine toenhance adhesion between the catalyst layer and the carbon paper,thereby obtaining a cathode. An anode was prepared using a commerciallyavailable PtRu Black catalyst and a unit cell was prepared using thecathode and the anode.

Performance Test of the Unit Cell

The performance of the unit cell obtained above was measured at 30° C.,40° C., and 50° C. while supplying 1 M methanol and air. The results areshown in FIG. 5. Although the fuel cell according to the presentinvention used the catalyst in a concentration of 1 mg/cm² or less, ithad the equivalent or better performance when compared to a conventionalfuel cell that uses a catalyst per unit area in a concentration of 2-4mg/cm².

As described above, a carbon nanosphere with at least one openingaccording to the present invention has a higher utilization of surfacearea and electrical conductivity and lower mass transfer resistance thana conventional carbon nanosphere or carbon nanotube, thus allowing forhigher current density and cell voltage with a smaller amount of metalcatalyst per unit area of a fuel cell electrode.

Further, the method for preparing a carbon nanosphere with at least oneopening according to the present invention provides a simpler and moreefficient method than a conventional method.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A carbon nanosphere having at least one opening, and having adiameter of about 100 nm to about 350 nm and a thickness of about 5 nmto about 30 nm.
 2. The carbon nanosphere of claim 1, wherein the openinghas a diameter of 5 nm or greater.
 3. The carbon nanosphere of claim 1,wherein the opening has a diameter of 5 nm to 150 nm.
 4. A carbonnanosphere having at least one opening, wherein the opening has adiameter of 5 nm or greater, and wherein the carbon nanosphere has athickness of about 5 nm to about 30 nm.
 5. The carbon nanosphere ofclaim 4, having a diameter of about 100 nm to about 350 nm.
 6. Thecarbon nanosphere of claim 4, wherein the opening has a diameter of 5 nmto 150 nm.